This invention relates to disk drives of the type in which a transducer carrying head rests on the disk surface when the disk is at rest and is supported on a film of fluid above the disk surface when the disk is rotating during operation. More particularly the invention is directed to the starting of disk operation when one or more of the transducer carrying heads may be adhered or stuck to the disk surface against which it rests when the drive is not operating.
A most common cause of disk drive failure is stiction, the adhesion of the transducer carrying head to disk surface upon which it rests when the drive is not operating. This occurs during the period between manufacture and initial operation and any prolonged periods between disk operation. The adhesion is a natural result induced by the ultra smooth surface finishes of both the slider air bearing surface and the disk recording surface upon which it rests. The stiction condition is made more severe by the presence of lubricant applied to the disk surface or contaminants that deposit on the disk surfaces whether or not a lubricant is used. The stiction problem is also aggravated as ever smoother surface finishes are provided on both the disk data surface and the head air bearing surfaces to enable the head to fly at only a few microinches above the disk and achieve higher linear storage densities. Simultaneously, the design objective is to reduce the size, weight and power consumption of the motor used to rotate the disk spindle assembly.
Various approaches have been used to overcome the stiction problem. A disk may be provided with a rougher surface in the zone where the head lands and is parked during periods of nonuse of the drive. U.S. Pat. No. 5,018,029 uses a solenoid to apply a shock force to the spindle to degrade the stiction at the time of starting.
U.S. Pat. No. 4,970,610 teaches the pulsing of the current supplied to the spindle motor at start at a frequency near the resonant frequency of the stuck system. This rapidly amplifies the torque such that the number of pulses in the sequence must be limited to prevent damage to the heads or suspensions. However, the pulse frequency and sequence are directed to the resonant frequency of a fully stuck system where all heads are adhered to the confronting disk surfaces and amplification is rapidly diminished as the applied pulse frequency departs from the resonant frequency in the environment where the stuck condition involves a few or less than all heads.
An effective way to apply a force to the stuck interface is through use of the voice coil motor that drives the actuator to move the head assembly from track to track and to maintain a selected head in alignment with an addressed track. The voice coil motor can apply greater torque since the objective of the actuator motor design is to have the greatest available torque available within the size and power restraints and the ability to limit any stray magnetic field that might interfere with other magnetic systems within the device.
U.S. Pat. No. 4,530,021 teaches the pulsing of the current to the voice coil motor to apply a short oscillatory force to the heads prior to the start of rotation. This technique may apply a torque of sufficient magnitude to damage the head-suspension gimbal support structure. U.S. Pat. No. 4,589,036 teaches the application of a current to the voice coil motor that would induce a constant velocity radial motion of the actuator assembly. When a back EMF is sensed, indicating a predetermined motion of the head assembly, the spindle motor is activated.